The advent of organic semiconductors has enabled fabrication on a flexible plastic substrate and thus realization of a flexible display. Since it was reported that a conjugated polymer exhibits semiconductor properties and an electrical conductivity comparable to that of a metal can be achieved by doping it, the organic semiconductors are being developed more actively.
Recently, many researches are being conducted on organic light emitting diodes that can be used for various mobile electronic devices and on organic thin-film transistors wherein even the devices for driving the light emitting diodes are replaced by organic materials.
The organic thin-film transistor is advantageous over the thin-film transistor using amorphous silicon or polysilicon in that it can be fabricated at low cost via a simple process and is excellently compatible with plastic substrates used to fabricate flexible displays. In particular, when an organic semiconductor material with superior solubility is used, a thin film can be prepared easily via a solution process, which allows for preparation with large area at much lower cost.
The existing general organic thin-film transistor has a structure of substrate/gate electrode/insulating layer/electrode (source and drain) layers/organic semiconductor layer. A gate electrode is formed on a substrate, an insulating layer is formed on the gate electrode, and an organic semiconductor layer, a source electrode and a drain electrode are formed sequentially on the insulating layer. When an organic semiconductor is used as an n-type semiconductor in the organic thin-film transistor having such a structure, if a low voltage is applied between the source and drain electrodes, a current proportional to the applied voltage flows. At this time, if a negative voltage is applied to the gate electrode, electrons having negative charge are forced to the upper organic semiconductor layer due to the electric field formed by the voltage. As a result, a depletion layer without conductive charges is formed near the insulating layer. In this situation, even when a voltage is applied between the source and drain electrodes, the current is low because of decreased amount of conductive charge carriers. Conversely, if a positive voltage is applied to the gate electrode, a negatively charged accumulation layer is formed near the insulating layer due to the electric field formed by the applied voltage. In this case, a large current can flow because there are many conductive charge carriers between the source and drain electrodes. Accordingly, the current flowing between the source and drain electrodes can be controlled by alternatingly applying negative and positive voltages to the gate electrode in the state where a voltage is applied between the source and drain electrodes.
In the existing organic thin-film transistor including the n-type semiconductor operating based on the above-described principle, a substrate and source, drain and gate electrodes with high thermal stability, an insulator with high insulating property (dielectric constant) and an organic semiconductor that allows facile charge transport are required. To overcome the problems of the organic thin-film transistor, there are much to be improved for the materials of the insulator, organic semiconductor, etc. In particular, since the organic semiconductor is the key material, to overcome the problem of the organic semiconductor material will provide a good solution to the problem of the organic thin-film transistor.
Until now, there has been more development in the p-type organic semiconductor materials based on hole transport than that of the n-type semiconductor materials based on electron transport. It is because, since hole mobility is larger than electron mobility in most cases, the n-type organic semiconductor tends to exhibit lower performance and conductivity as compared to the p-type organic semiconductor. Since fullerene (C60), which is the representative n-type organic semiconductor material, has low LUMO energy, it can transport electrons well. It is reported that an electron mobility up to 6 cm2/V·s can be achieved in vacuum. However, if an n-type transistor device is fabricated using the fullerene organic semiconductor material via a solution process, the electron mobility is very low as about 2.8×10−2 cm2/V·s (non-patent document 1).
Although an n-type polymer semiconductor having an electron mobility of 1 cm2/V·s was reported, the electron mobility is still far from commercialization (non-patent document 2). That is to say, there have been little development of a flexible, organic semiconductor material which has superior electron mobility and can be usefully used for a solution process.
To construct circuits such as a p-n junction diode, a bipolar transistor, a transducer, etc. using an organic material, the development of an n-type organic semiconductor material is necessary. Especially, the development of a novel n-type organic semiconductor material that can solve the problems of the existing art is necessary. A superior n-type organic semiconductor that can solve the above-described problems will be usefully applied not only in transistors but also as an electron acceptor material for organic thin-film solar cells.